For years, the gold standard for preventing running injuries has been centered on the “too much, too soon” theory, typically monitored through weekly training load metrics like the Acute:Chronic Workload Ratio (ACWR). Runners, coaches, and even wearable technology have long relied on these weekly averages to decide if a training volume increase is safe.

However, a groundbreaking new cohort study from Aarhus University, involving over 5,200 runners and analyzed by the RUNSAFE research group, suggests that everything we thought we knew about how running injuries develop might be wrong.

The Shift: From Weekly Averages to Single-Session Spikes

The study, published in the British Journal of Sports Medicine, indicates that the “too much” period for running injuries is likely much shorter than the one-week window traditionally used in wearable devices and research.

Instead of focusing on weekly volume, the researchers found that the risk of overuse injury is significantly tied to the distance of a single running session relative to the longest run a runner has completed in the past 30 days.

Key Findings:

• The 10% Threshold: If a single running session exceeds 10% of the longest run performed in the previous 30 days, the risk of sustaining a running-related overuse injury significantly increases.
• Dose-Response Relationship: The study identified a clear dose-response relationship:
  • Small spikes (>10% to 30%): 64% increased hazard of injury.
  • Moderate spikes (>30% to 100%): 52% increased hazard of injury.
  • Large spikes (>100%): A staggering 128% increased hazard of injury.
• Questioning Traditional Metrics: In contrast to the single-session findings, the researchers found that commonly used weekly metrics, such as the ACWR, did not show a positive association with injury risk, and in some cases, even showed an inverse association. Similarly, no significant relationship was identified for week-to-week workload ratios.

Implications for Wearables and Periodisation

This research suggests a potential paradigm shift in how we approach training load and periodisation.

• Rethinking Wearables: Current wearable devices often rely on weekly load calculations to guide progression. This study suggests these devices—and the runners using them—might need to pivot toward monitoring session-specific spikes to provide more precise guidance for injury prevention.
• New Periodisation Focus: Coaches and athletes are encouraged to look beyond total weekly mileage and pay closer attention to the intensity and distance of individual sessions. While the study highlights the 10% single-session rule, it also serves as a reminder that proper periodisation requires careful balancing of recovery and progressive overload to ensure the musculoskeletal system can tolerate applied loads.

The Bottom Line

While training errors remain a primary contributor to overuse injuries, it appears the “danger zone” isn’t necessarily just in your total weekly mileage—it’s in that single, unexpectedly long run that pushes your body beyond its current tolerance. To stay healthy, keep an eye on your longest run from the last month and avoid pushing too hard in your next single session.

This summary was generated with the assistance of Gemini based on the original article, with the aim of translating the research into insights for coaches, athletes and support staff.